Predistortion and postdistortion techniques, for cancelling the distortion introduced by the nonlinear transfer characteristic of electromagnetic devices, are well known in the art. Two examples of such circuits are given in U.S. Pat. Nos. 3,732,502 and 3,825,843. In each of the circuits disclosed in these patents, some means are provided for deliberately generating higher order signal components which are used to cancel the distortion signal components inadvertantly produced in an amplifier or other nominally linear electromagnetic device.
Advantageously, the distortion generator generates only selected higher order signal components, and does so over some prescribed frequency band of interest. In addition, the distortion generator should operate stably with time and temperature and be of simple design and easy to adjust.
In U.S. Pat. No. 3,732,502 the distortion generator comprises two RC coupled transistors which generate a multiplicity of higher order components. Means are provided for cancelling the first order signal component. However, there are no means for controlling the relative amplitudes of the higher order components or of eliminating any of them.
In U.S. Pat. No. 3,825,843, on the other hand, a more complicated circuit, including a squarer and a multiplier, is used to generate only the third order component. The problem with this circuit, however, is that it was found to be limited in available distortion power and relatively difficult to adjust over the wide frequency band of particular interest to applicant.